Proper soil maintenance is not difficult. It's much harder to stick to a plan than it is to understand what needs to be done.
To put the following advice in perspective, think of the world of horticulture 2,400 years ago.
Ancestors of all of the plants we now grow were growing with no “help” from humans and they did just fine. Things changed considerably beginning in the 19th century when plant hunters scoured remote regions of the world to send new species most notably to England where greenhouses and conservatories were built to show off exotic species.
In the case of roses, a huge number of species were collected from all parts of the Northern Hemisphere, the only place that roses naturally grow.
Before humans started moving plants from one location to another, roses grew just fine without irrigation or fertilization. In the course of 150 years of plant breeding, we have produced hybrid varieties that require extra applications of food and water to perform at their best. However, many varieties grow just fine in the right climates without adding food or water.
With two exceptions, San Diego's climate is not a right climate for roses in a natural state. Our region is home to only two species that most people have never seen, namely Rosa minutifolia, a rose that is not particularly attractive, and Rosa californica.
R. minutifolia grows in the most arid and hot climates of California and Baja California. The leaves are very tiny and the roots reportedly go down 20 feet. In San Diego County, R. californica is commonly found around Julian in streambeds that are wet in the winter. It is grown around the world.
In this climate, we have to feed and water our roses because garden roses have been bred to grow with vigor and bloom profusely. The price we pay for these improvements is the necessity of supplemental water and fertilizer.
Considering what it takes for native species to survive, it should come as no surprise that roses are not fussy and don't require any magic compound to grow. When was the last time you heard of a forester fertilizing a forest?
However, if you want the best from your roses, you will get much better results if you follow some basic rules of watering and fertilizing.
Feed the soil
Let's start with fertilizing. First, feed the soil rather than the rose. Most plants, including roses, grow best in organically alive soil. That is commonly ignored.
You may spend a lot of time encouraging big bushes that each year produce many cubic yards of plant growth. If you send this green waste to a landfill without adding an organic replacement to your soil you will gradually eliminate the organic matter there.
It is good to compost green waste that is not infected with fungus disease and recycle it in the garden. But most people have neither the service yard nor the time to do a good job of composting for a long time. If you can do a bit of composting, so much the better.
I have gardened on the same land for more than 40 years and have turned hard clay into very light organic soil. In the early years, I added about 10 yards of cow manure annually from local dairies, always following with an application of gypsum to counteract the sodium salt in the manure.
In recent years, I have changed to 6 yards a year of A-1 Lifelike from Hansen Aggregate and about 3 yards of composted green waste.
Forms of nitrogen
Roses need a large amount of nitrogen (N), phosphorous (P), and potassium (K). All fertilizers sold in California have NPK values in percentages conspicuously listed on the label.
Most rose growers place too much emphasis on these numbers. If a person describes a recommended fertilizer solely by these numbers, he knows just enough to be dangerous. Quite obviously a 4-6-8 is about the same as an 8-12-16 but half as strong as the latter. What is important is the chemical source of the components in the sack.
All amateur rose show exhibitors who are successful in competition have one thing in common: the use of high amounts of nitrogen.
There are some simple but crucial differences in the forms of nitrogen fertilizers. It comes in three forms: organic, ammonium and nitrates. Organics are nitrogen compounds derived from well rotted plant material and animal manures. They are plant material concentrated in form and supplemented with bacteria generally useful for soil tilth and plant growth.
Organics also include animal urine and most commonly in its synthetic form called urea.
Ammonium nitrogen is a form created when organic products are subjected to continuing heat, moisture and bacteria that changes its chemical content as can commonly be smelled in fresh animal manures.
Nitrates are sometimes found in further decomposition of an ammonium form of nitrogen but most commonly seen in mined minerals.
The most common form of nitrogen fertilizer is the synthetic product, urea. Urea has an NPK value of 45-0-0 which means that a 100-pound bag will contain 45 pounds of nitrogen and no phosphate or potassium compounds. Urea is found in virtually every all-purpose fertilizer compound. It is fast acting but will not burn the plant roots or foliage if applied in small quantities. It is quite inexpensive.
In the soil, organic fertilizers are converted by soil microbes to an ammonium form that the plant uses to a moderate degree and thence to a nitrate form that is repelled by soil particles but immediately taken up by plant roots if they are present.
I don't recommend the use of any nitrate form of fertilizer on roses because it causes uneven growth, meaning the plant will grow like mad for a few days after application and then revert to its baseline growth. Of equal importance is the fact that high levels of nitrate nitrogen produce excessive foliage at the expense of good flower production.
Sulfate of ammonia is an ammonium form of nitrate and is quite acidic. That is both good and bad. It is good because it can maintain a moderately acidic balance (pH 6.5) in San Diego's naturally alkaline soils that are made more alkaline by our water supplies.
I recommend use of sulfate of ammonia for a period of only two months in the early spring. I also recommend that it not be used on potted plants because of the light soil used in most potting mixes and the elevation of soil pH to unacceptable levels.
Most home gardeners don't have the means to use nitrate nitrogen properly. It is commonly used in greenhouse crops, constantly mixed with water at five parts per million and that is too tricky for most home gardeners.
Phosphate, a phosphorous compound, moves through the soil at the rate of about 2 inches a year. Thus when it is applied to the soil surface it will be consumed by surface weeds if they are present and will rarely benefit the rose. This fertilizer will not burn any plant part and it is best to add 1 cup to the bottom of a planting hole at the time of planting. It works best if not mixed with the soil.
Potassium is best supplied by sulfate of potash. This is acidic like sulfate of ammonia. Avoid using murate of potash which is found in many fertilizers intended to appeal to thrifty gardeners. Potassium compounds are not found in pure form in many nurseries but either sulfate of potash and murate of potash are found in most all-purpose fertilizers.
Murate will burn the foliage and stunt growth and is not appropriate for use in San Diego. You will not find pure sulfate of potash in some nurseries but most independent nurseries in San Diego stock it at a moderate price.
To add necessary trace elements to the soil I recommend the use of 1 tablespoon per plant of fish emulsion every other week from April through August. If you skip a week or two, that's all right.
I can spread the dry fertilizers on 300 roses in fewer than 20 minutes. The fertilizers I've recommended don't need to be scratched in because the products are highly soluble.
My wife fertilizes with fish emulsion because I don't like the smell. She uses a submersible electric pump placed in a 30 gallon trash can. It takes about two hours to do 300 roses from setup through cleanup.
Dick Streeper has written a monthly column on roses since 1981. He is a past president of the San Diego Rose Society and the East County Rose Society, a founder of the Inez Grant Parker Memorial Rose Garden in Balboa Park, and a past director of the American Rose Society. He answers rose questions at streeper@cox.net.
